The present application claims priority to Japanese Patent Application No. 2000-378502 filed Dec. 13, 2000, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an optical apparatus using an acoustooptic device.
2. Description of the Related Art
Various kinds of acoustooptic devices made of lithium niobate LiNbO3 or the like have been proposed. For example, U.S. Pat. No. 5,657,152 discloses an acoustooptic device in which on an LiNbO3 substrate, an optical waveguide layer having a higher light refractive index than the LiNbO3 substrate is formed and a Rayleigh wave, which is a kind of surface acoustic wave, is excited on the optical waveguide layer to thereby deflect light. U.S. Pat. No. 5,138,482 discloses a light modulator in which a light beam is divided into a plurality of beams by use of an optical waveguide made of a material capable of propagating surface elastic waves and the difference in modulation timing among a plurality of light beams is eliminated.
The above-described conventional optical devices use an interdigital transducer (hereinafter, abbreviated as IDT) to deflect light. The basic structure of the IDT is shown in FIG. 1. In the IDT 101, a comb-shaped electrode 102a having a plurality of linear electrode fingers 103a and a comb-shaped electrode 102b having a plurality of linear electrode fingers 103b are opposed to each other so that the electrode fingers 103a and 103b are interdigitated. An oscillator 104 applies a voltage of a predetermined frequency to the comb-shaped electrodes 102a and 102b. 
FIG. 2 shows the IDT 101 formed on an optical waveguide layer 105. When the voltage of the predetermined frequency is applied to the IDT 101 by the oscillator 104, a surface acoustic wave 106 is generated on the optical waveguide layer 105. When a light beam is incident on the optical waveguide layer 105, the surface acoustic wave 106 diffracts a light beam 107 as schematically shown in FIG. 3. The angle of diffraction depends on the frequency and the elastic modulus of the optical waveguide layer 105.
However, the conventional optical devices are capable of deflecting light only within the same plane.
A principal object of the present invention is to provide a diffraction optical device having a light condensing property like that of a lens by using an acoustooptic device.
Another object of the present invention is to provide an optical apparatus in which the focal length is easily adjusted.
These and other objects are attained by a diffraction optical device comprising an optical waveguide made of an acoustooptic material, an interdigital transducer provided on the optical waveguide and in which a plurality of arc-shaped electrode fingers are interdigitated so that the interdigital transducer is substantially circular in shape as a whole, and an oscillator device for applying a voltage of a predetermined frequency to the interdigital transducer.
The substantially circular shape includes a circle, an annular shape, and a circle and an annular shape a part of which is cut away.
The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description taken in conjunction with the accompanying drawings.